1. Field
The present invention relates generally to communications systems, and more specifically, to systems and techniques to measure relative power levels of channel allocations in transmitting devices.
2. Background
Recently, communications systems have been developed to allow the transmission of signals from an origination station to a physically distinct destination station. In transmitting signals from the origination station over a communication link, the signal is first converted into a form suitable for efficient transmission over the communication link. As used herein, the communication link comprises a media, over which a signal is transmitted. Conversion, or modulation, of the signal involves varying a parameter of a carrier wave in accordance with the signal in such a way that the spectrum of the resulting modulated carrier is confined within the communication link bandwidth. At the destination station, the original signal is replicated from a version of the modulated carrier received over the communication link. Such a replication is generally achieved by using an inverse of the modulation process employed by the origination station.
Modulation also facilitates multiple-access, i.e., simultaneous transmission and/or reception, of several signals over a common communication link. Numerous multiple-access techniques are known in the art, such as time division multiple-access (TDMA), frequency division multiple-access (FDMA), space division multiple-access, polarization division multiple-access, code division multiple-access (CDMA), and other similar multi-access techniques. The multiple-access concept is a channel allocation methodology which allows multiple user access to a common communication link. The channel allocations can take on various forms depending on the specific multi-access technique. By way of example, in FDMA systems, the total frequency spectrum is divided into a number of smaller sub-bands and each user is given its own sub-band to access the communication link. Alternatively, in TDMA systems, each user is given the entire frequency spectrum during periodically recurring time slots. In CDMA systems, each user is given the entire frequency spectrum for all of the time but distinguishes its transmission through the use of a unique pseudo-random code.
To minimize interference between channel allocations in multiple-access environments, various regulatory bodies have promulgated minimum performance standards for communications devices. Some multi-access schemes require that the transmission power for each channel be substantially the same. To ensure compliance with this requirement, numerous methodologies have been developed to measure the relative power levels of different channel allocations. Heretofore, relative power measurements in TDMA systems have been made using a spectrum analyzer set to the time domain. The spectrum analyzer has also been used to measure relative power levels in FDMA systems with the spectrum analyzer set to the frequency domain.
Relative power measurements in CDMA systems generally require that the desired channels be recovered from those undesired channels that share the same frequency spectrum. Channel recovery is made possible by transmitting each signal with a different pseudo-random binary sequence that modulates a carrier, and thereby, spreads the spectrum of the signal waveform. The transmitted signals are separated in the receiver by a correlator that uses a corresponding pseudo-random binary sequence to despread the desired signal's spectrum. The undesired signals, whose pseudo-random binary sequence do not match, are not despread in bandwidth and contribute only to noise.
Code domain power coefficient (ρi) calculations can be useful for despreading the spectrum of the CDMA channels of interest as well as measuring their relative power levels. The code domain power coefficient (ρi) is defined as the normalized ratio of the power of the actual waveform generated by the CDMA transmitter that correlates with the ideal waveform when modulated by the CDMA channel of interest and can be expressed as follows:
                              ρ          i                =                                                                                            ∑                                      k                    =                    1                                    N                                ⁢                                                      z                    k                                    ·                                      r                                          i                      ,                      k                                        *                                                                                      2                                              {                                                ∑                                      k                    =                    1                                    N                                ⁢                                                                                                z                      k                                                                            2                                            }                        ·                          {                                                ∑                                      k                    =                    1                                    N                                ⁢                                                                                                r                                              i                        ,                        k                                                                                                  2                                            }                                                          (        1        )            where:
i corresponds to the ith CDMA channel;
zk=z[k] which is the kth sample of the actual waveform;
ri,k=ri[k] which is the kth sample of the ideal waveform spread by the pseudo-random code for the ith CDMA channel;
N=the number of samples; and
[ ]* represents the complex conjugate.
The dot-product in the numerator between the actual waveform generated by the CDMA transmitter and the ideal waveform effectively despreads the spectrum of the signal for the CDMA channel of interest when the number of samples M is a multiple of the period of the spreading code. The denominator of equation (1) normalizes the computation. The difference in the code domain power coefficients for the CDMA channels of interest provides an indication of the difference in power.
Notwithstanding the existing methodologies for measuring the relative power of channel allocations in different multi-access schemes, the implementation of a common way for measuring the relative power of channels for different multi-access schemes is desirable. This is particularly true as more complex multi-access schemes become commonplace. By way of example, a CDMA system can be a hybrid of FDMA and CDMA techniques where the total system bandwidth is divided into a set of wideband channels, each of which contains several CDMA channels. Alternatively, the CDMA system can be a hybrid of TDMA and CDMA techniques where several CDMA channels are assigned to periodically recurring time slots, or one or more CDMA channels are partitioned into several TDMA channels. Based on hybrid channelization of the waveforms, relative power measurements using code domain power coefficients (ρi) may yield inaccurate results when several TDMA channels are contained in a single CDMA channel. The potential inaccuracy results from the normalization of the cross-correlated value between the actual waveform generated by the CDMA transmitter and the ideal waveform. Accordingly, there is a need for a more generalized methodology for measuring relative power that can be applied to a variety of different multi-access schemes.